Accuracy in placing a projectile onto a target using a rifle requires the shooter to determine three primary elements: 1) distance to the target, 2) the incline of the projectile as it leaves the rifle, and 3) the cant of the rifle at the moment the projectile leaves the rifle. The rifle cant is defined as the degree of rotational tilt the rifle has along the axis of the rifle barrel. Distance and incline are part of “sighting” a rifle to a target and generally require several elements that must be determined by the shooter. Since gravity tends to bring the projectile downward, an incline of the barrel must often be made to hit a target at some distance. Commonly used sights for setting the incline can be a groove or aperture at the rear end of the post or the point at the barrel end-muzzle. Once the shooter determines a point at which the projectile is aimed, the shooter uses one eye to align the post into the groove, which effectively aligns the rifle both horizontally and vertically to the point of aim.
However, such sighting means may not offer the shooter with the degree of accuracy that may be desired. To improve the accuracy of the horizontal and vertical alignment, some rifle assemblies make use of a magnification means, referred to as a scope. A scope typically provides the shooter with a glass view port displaying horizontal and vertical lines in addition to a magnified view of the point of aim. Scopes incorporate vertical and horizontal adjustment means. The shooter simply makes the calculated vertical and horizontal adjustments to account for situational issues such as wind, temperature, and distance and aligns the point of aim with the intersection of these two lines, commonly referred to as cross-hairs. Scopes can contain a system of lines, dots, cross hairs, wires, or electrically projected images which aid aligning the barrel to the point of aim. Scopes are generally mounted on top of the action assembly near the back end of the barrel of the rifle and are attached thereto with some means for adjustment. A common adjustment means is a ring and slotted bar-rail device also known as scope rings and scope bases. These adjustments are typically made at a shooting range or target practice area where the rifle is placed in a holder to ensure proper alignment and target distances are accurately known. Using this method, a rifle and scope can be adjusted to provide the shooter with a high degree of accuracy.
However, using a holder at a shooting range for calibrating a rifle's proper incline as a function of distance to the target often does not represent real world situations where the shooter is either standing or prone with the rifle being held at the time of firing. In these situations, the rifle is often twisted or rotated about the axis of the barrel. The physics of projectile firing is greatly affected by this degree of rotation or “cant” of the rifle at the moment of firing. For example, a left angle of cant tends to result in the shot being to the lower left of the point of aim. Shooters, especially competition target shooters, must compensate for the cant of the rifle to improve shot accuracy.
Various means have been presented in the prior art to provide feedback to the shooter of the degree of cant during their aim. One such example is U.S. Pat. No. 6,813,855 where Pinkley presents an apparatus where among other accompanying pieces, a bubble level is strapped to the rifle stock underneath the scope. Pinlkley's cant compensation method involves the steps of positioning the firearm and scope with a canted reticle system so that its vertical axis is positioned as indicated by the level bubble of the mounted level sub-assembly, positioning the vertical reference shaft sub-assembly a distance from the muzzle end of the firearm. The shooter then rotates the scope on the firearm sufficiently to align the vertical cross hair of the scope reticle system with the distant vertical reference shaft sub-assembly. Lastly, the shooter locks the sighting scope in the corrected position on the firearm.
The prior art attempts to provide the shooter with feedback for the cant of the rifle tend to be attached to the scope and as such are accessories that must be carefully assembled to the scope and are not suitable for shooting situations where speed and durability are required. Also, the prior art cant measurement systems themselves must be thoroughly tested and calibrated by the shooter so that typically only that shooter, with that cant feedback device, on that special rifle, and carefully calibrated by a trained technician can be used to produce the degree of accuracy in critical or competitive shooting environments.
It has been discovered that by locating a pre-calibrated bubble level between the shooter's eye and the scope and carefully machining the bubble level within the rifle system, a reliable cant feedback system can be readily made available to any shooter and repeatable across an entire weapon platform. Also, by carefully embedding the bubble level within the body of the rifle, the cant feedback method can be durable and repeatable for a whole range of shooting applications, especially for the war-fighter.
Additionally, by embedding the cant feedback means into the rifle's stock and providing an accurate measurement of the rifle's cant, calibrating the firearm is greatly facilitated. By placing a plumb line at the desired target calibration distance (100 yards or 100 meters, for example), and then aligning the vertical reticle of the scope with the rifle at zero cant, the scope reticle-aiming reference are “trued” to the cant axis of the rifle. Once this initial process is completed, the rifle and scope are now calibrated for a “zero-cant” condition relative to each other.